A bundle of tube-shaped semi-permeable membranes, when perfused with tissue culture medium simulates the in vivo capillary bed. Isolated cells injected into the extracapillary space receive nutrients and have cell products removed by diffusion through the capillary walls. In this nearly-physiologic environment the cells form solid masses of tissue. The relationship of solid, hormone-producing pituitary tumors and mammary carcinomata is being studied, using this technique, to determine the mechanisms of hormone-dependent tumor growth and regression.